Layered body, and saw device

ABSTRACT

A ceramic substrate is formed of polycrystalline ceramic and has a supporting main surface. At the supporting main surface of the ceramic substrate, the mean of grain sizes of the polycrystalline ceramic is 15 μm or more and less than 40 μm and the standard deviation of the grain sizes is less than 1.5 times the mean.

TECHNICAL FIELD

The present disclosure relates to a ceramic substrate, a layered body,and a SAW device.

The present application claims the priority based on Japanese PatentApplication No. 2017-198780 filed on Oct. 12, 2017, the entire contentsof which are incorporated herein by reference.

BACKGROUND ART

SAW devices (surface acoustic wave devices) are installed incommunication apparatuses such as cellular phones in order to removenoises included in electrical signals. SAW devices have a structure inwhich electrodes are formed on a piezoelectric substrate. To radiateheat during operation, the piezoelectric substrate is disposed on a basesubstrate formed of a material with good heat radiation properties.

For example, a substrate formed of single-crystalline sapphire can beemployed as the base substrate. However, if such a substrate formed ofsingle-crystalline sapphire is employed as the base substrate, theproduction cost of SAW devices increases. To address this problem, therehas been proposed a SAW device having a structure in which a ceramicsubstrate formed of polycrystalline spinel is employed as a basesubstrate, and a piezoelectric substrate and the ceramic substrate arebonded to each other through Van der Waals force (e.g., refer to PTL 1).

CITATION LIST Patent Literature

-   PTL 1: Japanese Unexamined Patent Application Publication No.    2011-66818

SUMMARY OF INVENTION

The ceramic substrate according to the present disclosure is a ceramicsubstrate formed of polycrystalline ceramic and having a supporting mainsurface. At the supporting main surface, the mean of grain sizes of thepolycrystalline ceramic is 15 μm or more and less than 40 μm and thestandard deviation of the grain sizes is less than 1.5 times the mean.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view illustrating a structure of alayered body including a ceramic substrate and a piezoelectricsubstrate.

FIG. 2 is a schematic plan view illustrating a supporting main surfaceof a ceramic substrate.

FIG. 3 is a flowchart schematically illustrating a method for producinga ceramic substrate, a layered body, and a SAW device.

FIG. 4 is a schematic sectional view for describing the method forproducing a layered body and a SAW device.

FIG. 5 is a schematic sectional view for describing the method forproducing a layered body and a SAW device.

FIG. 6 is a schematic sectional view for describing the method forproducing a layered body and a SAW device.

FIG. 7 is a schematic view for describing the method for producing alayered body and a SAW device.

FIG. 8 is a schematic view illustrating a structure of a SAW device.

DESCRIPTION OF EMBODIMENTS Problems to be Solved by Present Disclosure

When a ceramic substrate and a piezoelectric substrate have aninsufficient bonding strength, the ceramic substrate and thepiezoelectric substrate are separated from each other in the productionprocess of SAW devices, which decreases the yield in the production ofSAW devices. To further reduce the production cost of SAW devices, thebonding strength between the ceramic substrate and the piezoelectricsubstrate needs to be further increased.

Accordingly, it is an object to provide a ceramic substrate that can bebonded to a piezoelectric substrate with a sufficient bonding strength,and a layered body and a SAW device including the ceramic substrate.

Advantageous Effects of Present Disclosure

According to the ceramic substrate of the present disclosure, a ceramicsubstrate that can be bonded to a piezoelectric substrate with asufficient bonding strength can be provided.

Description of Embodiments of Present Disclosure

First, embodiments of the present disclosure will be listed anddescribed. The ceramic substrate according to the present disclosure isa ceramic substrate formed of polycrystalline ceramic and having asupporting main surface. In this ceramic substrate, at the supportingmain surface, the mean of grain sizes of the polycrystalline ceramic is15 μm or more and less than 40 μm and the standard deviation of thegrain sizes is less than 1.5 times the mean.

According to studies conducted by the present inventors, the bondingstrength between the ceramic substrate and the piezoelectric substratecan be improved by controlling the mean of grain sizes at the supportingmain surface of the ceramic substrate to 15 μm or more and less than 40μm. However, even if the mean of grain sizes at the supporting mainsurface is 15 μm or more and less than 40 μm, the ceramic substrate andthe piezoelectric substrate sometimes have an insufficient bondingstrength. As a result of further studies on this cause, it is found thateven if the mean of grain sizes at the supporting main surface is 15 μmor more and less than 40 μm, the ceramic substrate and the piezoelectricsubstrate have an insufficient bonding strength when the variation ingrain size is large, more specifically, when the standard deviation ofthe grain sizes is more than or equal to 1.5 times the mean. Therefore,in addition to controlling the mean of grain sizes at the supportingmain surface to 15 μm or more and less than 40 μm, the standarddeviation of the grain sizes needs to be less than 1.5 times the mean tobond the ceramic substrate and the piezoelectric substrate to each otherwith a sufficient bonding strength.

In the ceramic substrate according to the present disclosure, the meanof grain sizes of the polycrystalline ceramic at the supporting mainsurface is 15 μm or more and less than 40 μm and the standard deviationof the grain sizes is less than 1.5 times the mean. As a result,according to the ceramic substrate of the present disclosure, a ceramicsubstrate can be bonded to a piezoelectric substrate with a sufficientbonding strength.

In the above ceramic substrate, the residual stress at the supportingmain surface may be −300 MPa or more and 300 MPa or less. In theproduction process of SAW devices, a layered body in which the ceramicsubstrate and the piezoelectric substrate are bonded to each other issubjected to a heat cycle including heating and cooling. This may resultin an insufficient bonding strength between the ceramic substrate andthe piezoelectric substrate. When the absolute value of the residualstress at the supporting main surface is 300 MPa or less, the sufficientbonding strength is easily maintained even if such a heat cycle isapplied. For the residual stress, a negative value indicates compressivestress and a positive value indicates tensile stress. The residualstress can be measured with, for example, an X-ray diffractometer.

The above ceramic substrate may be formed of at least one materialselected from the group consisting of spinel (MgAl₂O₄), alumina (Al₂O₃),magnesia (MgO), silica (SiO₂), mullite (3Al₂O₃.2SiO₂), cordierite(2MgO.2Al₂O₃.5SiO₂), calcia (CaO), titania (TiO₂), silicon nitride(Si₃N₄), aluminum nitride (AlN), and silicon carbide (SiC). Thesematerials are suitable as materials for the ceramic substrate accordingto the present disclosure. Among these materials, spinel is preferred.

A layered body according to the present disclosure includes the aboveceramic substrate according to the present disclosure and apiezoelectric substrate formed of a piezoelectric material and having abonding main surface. The supporting main surface of the ceramicsubstrate and the bonding main surface of the piezoelectric substrateare bonded to each other through Van der Waals force. The layered bodyaccording to the present disclosure includes the ceramic substrateaccording to the present disclosure. Therefore, according to the layeredbody of the present disclosure, the ceramic substrate and thepiezoelectric substrate can be bonded to each other with a sufficientbonding strength.

In the above layered body, the piezoelectric substrate may be formed oflithium tantalate (LiTaO₃) or lithium niobate (LiNbO₃). These materialsare suitable as materials for the piezoelectric substrate in the layeredbody according to the present disclosure.

A SAW device according to the present disclosure includes the layeredbody according to the present disclosure and an electrode formed on amain surface of the piezoelectric substrate, the main surface beinglocated opposite to the ceramic substrate. The SAW device according tothe present disclosure includes the ceramic substrate according to thepresent disclosure. Therefore, according to the SAW device of thepresent disclosure, a SAW device in which the ceramic substrate and thepiezoelectric substrate are bonded to each other with a sufficientbonding strength can be provided.

Details of Embodiments of Present Disclosure

Next, a ceramic substrate and a layered body according to embodiments ofthe present disclosure will be described with reference to the attacheddrawings. In the drawings, the same or corresponding parts aredesignated by the same reference numerals, and the description thereofis omitted.

Referring to FIG. 1 and FIG. 2 , a ceramic substrate 10 according tothis embodiment is formed of polycrystalline ceramic and has asupporting main surface 11 for supporting a piezoelectric substrate 20that is another substrate. That is, the ceramic substrate 10 is anaggregate of many grains 10A. As illustrated in FIG. 2 , many grains 10Aare exposed at the supporting main surface 11. At the supporting mainsurface 11, the mean of diameters (grain sizes) of the grains 10A is 15μm or more and less than 40 μm, and the standard deviation of thediameters is less than 1.5 times the mean. The grain size of each of thegrains 10A can be determined by, for example, the following method.First, the supporting main surface 11 is observed with a microscope tomeasure the area of the grain 10A. Then, the diameter of a circle havingthe measured area is defined as a grain size. The mean of grain sizescan be determined by, for example, observing a plurality of regions ofthe supporting main surface 11 using a microscope and calculating thearithmetic mean of grain sizes in the regions.

Referring to FIG. 1 , a layered body 1 according to this embodimentincludes the ceramic substrate 10 and the piezoelectric substrate 20.The piezoelectric substrate 20 is formed of a single-crystallinepiezoelectric material such as single-crystalline lithium tantalate orsingle-crystalline lithium niobate. The ceramic substrate 10 is formedof a polycrystalline ceramic made of at least one material selected fromthe group consisting of spinel, alumina, magnesia, silica, mullite,cordierite, calcia, titania, silicon nitride, aluminum nitride, andsilicon carbide and is preferably formed of a polycrystalline ceramicmade of any one of the foregoing materials.

The piezoelectric substrate 20 has an exposed main surface 21 that isone main surface and a bonding main surface 22 that is a main surfaceopposite to the exposed main surface 21. The piezoelectric substrate 20is disposed on the supporting main surface 11 of the ceramic substrate10 so that the bonding main surface 22 is in contact with the supportingmain surface 11. The ceramic substrate 10 and the piezoelectricsubstrate 20 are bonded to each other through Van der Waals force.

For the ceramic substrate 10, the mean of grain sizes of thepolycrystalline ceramic at the supporting main surface 11 is 15 μm ormore and less than 40 μm, and the standard deviation of the grain sizesis less than 1.5 times the mean. Therefore, the ceramic substrate 10 isa ceramic substrate that can be bonded to the piezoelectric substrate 20with a sufficient bonding strength. The layered body 1 includes theceramic substrate 10. Therefore, the layered body 1 is a layered body inwhich the ceramic substrate 10 and the piezoelectric substrate 20 arebonded to each other with a sufficient bonding strength.

For the ceramic substrate 10, the residual stress at the supporting mainsurface 11 is preferably −300 MPa or more and 300 MPa or less. When theabsolute value of the residual stress at the supporting main surface 11is 300 MPa or less, a sufficient bonding strength between the ceramicsubstrate 10 and the piezoelectric substrate 20 can be maintained evenif a heat cycle is applied in the production process of SAW devices. Theresidual stress at the supporting main surface 11 is more preferably−200 MPa or more and 200 MPa or less and further preferably −100 MPa ormore and 100 MPa or less.

At the supporting main surface 11 of the ceramic substrate 10, thestandard deviation of the grain sizes is more preferably less than 1time the mean. Thus, the ceramic substrate 10 and the piezoelectricsubstrate 20 can be bonded to each other with a sufficient bondingstrength with more certainty.

Next, a method for producing a ceramic substrate 10, a layered body 1,and a SAW device 100 according to this embodiment will be described.Referring to FIG. 3 , the method for producing a ceramic substrate 10, alayered body 1, and a SAW device 100 according to this embodimentincludes a substrate providing step performed first as a step (S10). Inthe step (S10), referring to FIG. 4 , a ceramic substrate 10 formed of apolycrystalline ceramic made of at least one material selected from thegroup consisting of spinel, alumina, magnesia, silica, mullite,cordierite, calcia, titania, silicon nitride, aluminum nitride, andsilicon carbide is provided. For example, a ceramic substrate 10 formedof a polycrystalline ceramic made of one material selected from theabove group is provided. Specifically, for example, when a ceramicsubstrate 10 formed of polycrystalline spinel is provided, a rawmaterial powder is provided by mixing a magnesia powder and an aluminapowder, and a molded body is produced by molding the raw materialpowder. The molded body can be produced by, for example, performingpreforming by press molding and then performing CIP (cold isostaticpressing).

Subsequently, a sintering process is performed on the molded body. Thesintering process can be performed by a method such as vacuum sinteringor HIP (hot isostatic pressing). Thus, a sintered body is obtained. Thesintered body is then sliced to obtain a ceramic substrate 10 having adesired shape (thickness) (refer to FIG. 4 ). Herein, the size of thegrains 10A and the variation in size can be adjusted within the desiredrange by controlling the heating rate during sintering, the sinteringtemperature, and the holding time during sintering. Specifically, theheating rate can be set to, for example, 0.5° C./min or more and 5°C./min or less. The sintering temperature can be set to, for example,1500° C. or higher and 2000° C. or lower. The holding time duringsintering can be set to, for example, 2 hours or longer and 10 hours orshorter. By controlling the heating rate, the sintering temperature, andthe sintering time, the mean of grain sizes is easily controlled to 15μm or more and less than 40 μm, and the standard deviation of the grainsizes is easily controlled to less than 1.5 times the mean.

Subsequently, a rough polishing step is performed as a step (S20). Inthe step (S20), referring to FIG. 4 , rough polishing treatment isperformed on the supporting main surface 11 of the ceramic substrate 10provided in the step (S10).

Subsequently, an annealing step is performed as a step (S30). In thestep (S30), annealing is performed on the ceramic substrate 10.Specifically, for example, the ceramic substrate polished in the step(S20) is heated to a temperature range of 1000° C. or higher and 1500°C. or lower and held for 2 hours or longer and 10 hours or shorter. Thisdecreases the residual stress introduced to the ceramic substrate 10 inthe steps (S10) and (S20). As a result, the residual stress at thesupporting main surface 11 is easily controlled to −300 MPa or more and300 MPa or less.

Subsequently, a finishing polishing step is performed as a step (S40).In the step (S40), referring to FIG. 4 , finishing polishing treatmentis performed on the supporting main surface 11 of the ceramic substrate10 annealed in the step (S30). Thus, a ceramic substrate 10 according tothis embodiment is completed.

Subsequently, a bonding step is performed as a step (S50). In the step(S50), the ceramic substrate 10 subjected to finishing polishing in thestep (S40) and a separately provided piezoelectric substrate 20 formedof lithium tantalate or lithium niobate are bonded to each other.Specifically, for example, the ceramic substrate 10 and thepiezoelectric substrate 20 are washed, dried, and then inserted into achamber, and the pressure in the chamber is reduced. As indicated byarrows in FIG. 4 , the supporting main surface 11 and the bonding mainsurface 22 are irradiated with, for example, Ar (argon) beams. Thiscleans the supporting main surface 11 of the ceramic substrate 10 andthe bonding main surface 22 of the piezoelectric substrate 20. Then, theceramic substrate 10 and the piezoelectric substrate 20 are bonded toeach other so that the bonding main surface 22 of the piezoelectricsubstrate 20 is in contact with the supporting main surface 11 of theceramic substrate 10. Thus, the ceramic substrate 10 and thepiezoelectric substrate 20 are bonded to each other through Van derWaals force. As a result, a layered body 1 according to this embodimentis obtained.

Next, a method for producing a SAW device including the layered body 1including the ceramic substrate 10 will be described. Referring to FIG.3 , after the step (S50), a thickness decreasing step is performed as astep (S60). In the step (S60), referring to FIG. 1 and FIG. 5 , thethickness of the piezoelectric substrate 20 of the layered body 1obtained in the step (S50) is decreased. Specifically, for example, anexposed main surface 21 of the piezoelectric substrate 20 is subjectedto grinding treatment. Thus, the thickness of the piezoelectricsubstrate 20 is decreased to a thickness appropriate for SAW devices.

Subsequently, an electrode forming step is performed as a step (S70). Inthe step (S70), referring to FIG. 5 to FIG. 7 , comb-shaped electrodesare formed on the exposed main surface 21 of the piezoelectric substrate20. FIG. 6 is a sectional view taken along line VI-VI in FIG. 7 .Specifically, referring to FIG. 6 and FIG. 7 , a conductor film made ofan electric conductor such as Al is formed on the exposed main surface21 of the piezoelectric substrate 20 whose thickness has beenappropriately adjusted in the step (S60). The conductor film can beformed by, for example, a sputtering method. A resist is then appliedonto the conductor film to form a resist film. By performing exposureand development, an opening is formed in a region other than regionscorresponding to desired shapes of input-side electrodes 30 andoutput-side electrodes 40. For example, wet etching is performed usingthe resist film having the opening formed therein as a mask to form aplurality of pairs of input-side electrodes 30 and output-sideelectrodes 40 as illustrated in FIG. 6 and FIG. 7 . FIG. 6 and FIG. 7illustrate a region corresponding to a pair of input-side electrode 30and output-side electrode 40. The electrode interval of the comb-shapedelectrode in the input-side electrode 30 and the output-side electrode40 can be appropriately determined in accordance with the frequency ofsignals to be output.

Subsequently, a chip forming step is performed as a step (S80). In thestep (S80), the layered body 1 on which a plurality of pairs ofinput-side electrodes 30 and output-side electrodes 40 have been formedis cut in a thickness direction into a plurality of chips each includinga pair of input-side electrode 30 and output-side electrode 40.

Referring to FIG. 7 and FIG. 8 , an input-side wiring line 51 and anoutput-side wiring line 61 are then formed on the chip produced in thestep (S80) to complete a SAW device 100 (SAW filter) according to thisembodiment.

In the production method of the SAW device 100 according to thisembodiment, the mean of grain sizes of the polycrystalline ceramic atthe supporting main surface 11 of the ceramic substrate 10 is 15 μm ormore and less than 40 μm and the standard deviation of the grain sizesis less than 1.5 times the mean. Therefore, the ceramic substrate 10 andthe piezoelectric substrate 20 are bonded to each other with asufficient bonding strength. This suppresses the separation between theceramic substrate and the piezoelectric substrate in the productionprocess of SAW devices, which increases the yield in the production ofSAW devices. Note that another annealing step may be performed againafter the step (S40) from the viewpoint of further decreasing theabsolute value of the residual stress at the supporting main surface 11.

Referring to FIG. 8 , the SAW device 100 according to this embodimentincludes the layered body 1 including the ceramic substrate 10 and thepiezoelectric substrate 20 that bond to each other through Van der Waalsforce, the input-side electrode 30 and the output-side electrode 40,which are a pair of comb-shaped electrodes formed so as to be in contactwith the exposed main surface 21 of the piezoelectric substrate 20, theinput-side wiring line 51 connected to the input-side electrode 30, andthe output-side wiring line 61 connected to the output-side electrode40.

The input-side electrode 30 includes a first portion 31 and a secondportion 32. The first portion 31 includes a linear base portion 31A anda plurality of linear protrusions 31B that protrude from the baseportion 31A in a direction perpendicular to the direction in which thebase portion 31A extends. The second portion 32 includes a linear baseportion 32A that extends in parallel with the base portion 31A and aplurality of linear protrusions 32B that protrude from the base portion32A in a direction perpendicular to the direction in which the baseportion 32A extends and that fit into gaps between adjacent protrusions31B. The protrusions 31B and the protrusions 32B are disposed atpredetermined evenly spaced intervals.

The output-side electrode 40 includes a first portion 41 and a secondportion 42. The first portion 41 includes a linear base portion 41A anda plurality of linear protrusions 41B that protrude from the baseportion 41A in a direction perpendicular to the direction in which thebase portion 41A extends. The second portion 42 includes a linear baseportion 42A that extends in parallel with the base portion 41A and aplurality of linear protrusions 42B that protrude from the base portion42A in a direction perpendicular to the direction in which the baseportion 42A extends and that fit into gaps between adjacent protrusions41B. The protrusions 41B and the protrusions 42B are disposed atpredetermined evenly spaced intervals.

When an AC voltage serving as an input signal is applied to theinput-side electrode 30 through the input-side wiring line 51, a surfaceacoustic wave is generated on the exposed main surface 21 (surface) ofthe piezoelectric substrate 20 because of a piezoelectric effect, andthe surface acoustic wave propagates to the output-side electrode 40.Herein, the input-side electrode 30 and the output-side electrode 40have a comb shape as illustrated in FIG. 8 , and the protrusions 31B andthe protrusions 32B are evenly spaced and the protrusions 41B and theprotrusions 42B are evenly spaced. Thus, in a direction from theinput-side electrode 30 toward the output-side electrode 40, regions inwhich electrodes are formed on the exposed main surface 21 of thepiezoelectric substrate 20 are present at a predetermined period(electrode period). Therefore, the surface acoustic wave generated bythe input signal is excited most when its wavelength is coincident withthe electrode period, and is attenuated as the difference between thewavelength and the electrode period increases. Consequently, only asignal with a wavelength close to the electrode period is output throughthe output-side electrode 40 and the output-side wiring line 61.

In the above operation, the temperature of the piezoelectric substrate20 increases. In the SAW device 100 according to this embodiment, theceramic substrate 10 made of a material having good heat radiationproperties is disposed so as to be in contact with the piezoelectricsubstrate 20. Therefore, the SAW device 100 has high reliability.Furthermore, since the SAW device 100 includes the ceramic substrate 10according to this embodiment, the separation between the ceramicsubstrate 10 and the piezoelectric substrate 20 is suppressed in theproduction process. Consequently, the SAW device 100 can be producedwhile a high yield is maintained.

EXAMPLES

Samples of 11 ceramic substrates (spinel substrates) having differentmeans and standard deviations of grain sizes at a supporting mainsurface and different residual stresses at the supporting main surfacewere provided (sample Nos. 1 to 11). After the steps (S10) to (S50) ofthe above embodiment were performed using the samples, the bondingstrength between the ceramic substrate and the piezoelectric substratein the layered body 1 was evaluated by a crack opening method.Furthermore, after the samples were subjected to a heat cycle includingheating from room temperature to 300° C. and then cooling to roomtemperature, the bonding strength was evaluated in the same manner.

The grain size was measured by observing a polished supporting mainsurface with a microscope ECLIPSE LV100 manufactured by Nikon Corp. Themean and standard deviation of grain sizes were calculated using imageprocessing software included with the microscope. The residual stress atthe supporting main surface was measured by X-ray diffraction stressmeasurement. The X-ray used was Cu-Kα line focus. The excitationconditions were 45 kV and 40 mA. The scanning method was a sin²Ψ method(ISO-inclination method). The measurement range was 2θ=93° to 95.5°. Thestep size was 0.03°. The Ψ conditions were 13 levels (6 levels onpositive side, one level at zero, 6 levels on negative side){0≤sin²Ψ≤0.5}. The integration time was 1 or 3 seconds. The measurementplane was an MgAl₂O₄ (731) plane. Table 1 shows the experimentalresults.

TABLE 1 Standard Mean deviation Bonding μ of σ of Residual strengthgrain sizes grain sizes stress Bonding after heat No. (μm) (μm) σ/μ(MPa) strength cycle 1 12 10 0.83 −100 C C 2 15 25 1.67 100 C C 3 15 201.33 450 A B 4 15 15 1.00 −250 A A 5 25 40 1.60 −150 C C 6 25 30 1.20−350 A B 7 25 35 1.40 300 A A 8 37 60 1.62 −50 C C 9 37 40 1.08 −550 A B10 37 40 1.08 −150 A A 11 42 40 0.95 −100 C C

In Table 1, the bonding strength and the bonding strength after heatcycle were evaluated with grades of A: 1.0 J/m² or more, B: 0.5 J/m² ormore and less than 1.0 J/m², and C: less than 0.5 J/m².

Table 1 shows that the samples 1 and 11 in which the mean of grain sizesat the supporting main surface is outside the range of 15 μm or more andless than 40 μm have an evaluation result of C in terms of bondingstrength. This supports that the mean of grain sizes at the supportingmain surface needs to be in the range of 15 μm or more and less than 40μm. Even if the mean of grain sizes is within the range of 15 μm or moreand less than 40 μm, the samples (samples 2, 5, and 8) in which theratio (σ/μ) of the standard deviation of grain sizes to the mean ofgrain sizes is 1.5 or more have an evaluation result of C in terms ofbonding strength. This shows that to achieve a sufficient bondingstrength, the standard deviation of grain sizes needs to be less than1.5 times the mean in addition to the above mean condition.

Furthermore, even if the above-described mean and standard deviationconditions are satisfied, the samples (samples 3, 6, and 9) in which theresidual stress at the supporting main surface is outside the range of−300 MPa or more and 300 MPa or less have an evaluation result of B interms of bonding strength after heat cycle whereas the samples in whichthe residual stress is within the range of −300 MPa or more and 300 MPaor less have an evaluation result of A in terms of bonding strengthafter heat cycle. This confirms that when the absolute value of theresidual stress at the supporting main surface is 300 MPa or less, thebonding strength after heat cycle is improved.

The embodiments and Examples disclosed herein are mere examples in allrespects and should be understood as being non-limitative in anyperspective. The scope of the present invention is defined not by theabove description but by Claims. The scope of the present invention isintended to embrace all the modifications within the meaning and rangeof equivalency of the Claims.

The invention claimed is:
 1. A layered body comprising: a ceramicsubstrate formed of polycrystalline ceramic and having a supporting mainsurface; and a piezoelectric substrate formed of a piezoelectricmaterial and having a bonding main surface in contact with thesupporting main surface of the ceramic substrate, wherein at thesupporting main surface, a mean of grain sizes of the polycrystallineceramic is 15 μm or more and less than 40 μm, and a standard deviationof the grain sizes is less than 1.5 times the mean.
 2. The layered bodyaccording to claim 1, wherein a residual stress at the supporting mainsurface is −300 MPa or more and 300 MPa or less.
 3. The layered bodyaccording to claim 1, wherein the ceramic substrate is formed of atleast one material selected from the group consisting of spinel,alumina, magnesia, silica, mullite, cordierite, calcia, titania, siliconnitride, aluminum nitride, and silicon carbide.
 4. The layered bodyaccording to claim 1, wherein the polycrystalline ceramic is formed ofspinel.
 5. The layered body according to claim 1, wherein a residualstress at the supporting main surface is −300 MPa or more and 300 MPa orless, and the polycrystalline ceramic is formed of spinel.
 6. Thelayered body according to claim 1, wherein the supporting main surfaceof the ceramic substrate and the bonding main surface of thepiezoelectric substrate are bonded to each other through Van der Waalsforce.
 7. The layered body according to claim 6, wherein thepiezoelectric substrate is formed of lithium tantalate or lithiumniobate.
 8. A SAW device comprising: the layered body according to claim6; and an electrode formed on a main surface of the piezoelectricsubstrate, the main surface being located opposite to the ceramicsubstrate.